REM sleep and memory processing

The role of rapid eye movement sleep for amygdala-related memory processing.
Genzel L, Spoormaker V, Konrad BN, Dresler M.
Neurobiol Learn Mem. 2015 Jan 28.

Over the years, rapid eye movement (REM) sleep has been associated with general memory consolidation, specific consolidation of perceptual, procedural, emotional and fear memories, brain maturation and preparation of waking consciousness. More recently, some of these associations (e.g. general and procedural memory consolidation) have been shown to be unlikely, while others (e.g. brain maturation and consciousness) remain inconclusive. In this review, we argue that both behavioral and neurophysiological evidence supports a role of REM sleep for amygdala-related memory processing: the amygdala-hippocampus-medial prefrontal cortex network involved in emotional processing, fear memory and valence consolidation shows strongest activity during REM sleep, in contrast to the hippocampus-medial prefrontal cortex only network which is more active during non-REM sleep. However, more research is needed to fully understand the mechanisms.

Emotionally salient information and memory performance

Prospection and emotional memory: how expectation affects emotional memory formation following sleep and wake.
Tony J. Cunningham, Alexis M. Chambers, and Jessica D. Payne
Front Psychol. 2014 Aug 4;5:862. 

Successful prospective memory is necessarily driven by an expectation that encoded information will be relevant in the future, leading to its preferential placement in memory storage. Like expectation, emotional salience is another type of cue that benefits human memory formation. Although separate lines of research suggest that both emotional information and information explicitly expected to be important in the future benefit memory consolidation, it is unknown how expectation affects the processing of emotional information and whether sleep, which is known to maximize memory consolidation, plays a critical role. The purpose of this study was to investigate how expectation would impact the consolidation of emotionally salient content, and whether this impact would differ across delays of sleep and wake. Participants encoded scenes containing an emotionally charged negative or neutral foreground object placed on a plausible neutral background. After encoding, half of the participants were informed they would later be tested on the scenes (expected condition), while the other half received no information about the test (unexpected condition). At recognition, following a 12-h delay of sleep or wakefulness, the scene components (objects and backgrounds) were presented separately and one at a time, and participants were asked to determine if each component was old or new. Results revealed a greater disparity for memory of negative objects over their paired neutral backgrounds for both the sleep and wake groups when the memory test was expected compared to when it was unexpected, while neutral memory remained unchanged. Analyzing each group separately, the wake group showed a threefold increase in the magnitude of this object/background trade-off for emotional scenes when the memory test was expected compared to when it was unexpected, while those who slept performed similarly across conditions. These results suggest that emotional salience and expectation cues interact to benefit emotional memory consolidation during a delay of wakefulness. The sleeping brain, however, may automatically tag emotionally salient information as important, such that explicit instruction of an upcoming memory test does not further improve memory performance.

Sleep consolidates memories for competing tasks

Sleep Consolidation of Interfering Auditory Memories in Starlings
Brawn TP, Nusbaum HC, Margoliash D.
Psychological Science 2013, published on Feb 22

Memory consolidation has been described as a process to strengthen newly formed memories and to stabilize them against interference from similar learning experiences. Sleep facilitates memory consolidation in humans, improving memory performance and protecting against interference encountered after sleep. The European starling, a songbird, has also manifested sleep-dependent memory consolidation when trained on an auditory-classification task. Here, we examined how memory for two similar classification tasks is consolidated across waking and sleep in starlings. We demonstrated for the first time that the learning of each classification reliably interferes with the retention of the other classification across waking retention but that sleep enhances and stabilizes the memory of both classifications even after performance is impaired by interference. These observations demonstrate that sleep consolidation enhances retention of interfering experiences, facilitating opportunistic daytime learning and the subsequent formation of stable long-term memories

Sleep enhances emotional memories, preserves emotional intensity

Processing of emotional reactivity and emotional memory over sleep.
Baran B, Pace-Schott EF, Ericson C, Spencer RM.
Journal of Neuroscience. 2012 Jan 18;32(3):1035-42. 

Sleep enhances memories, particularly emotional memories. As such, it has been suggested that sleep deprivation may reduce posttraumatic stress disorder. This presumes that emotional memory consolidation is paralleled by a reduction in emotional reactivity, an association that has not yet been examined. In the present experiment, we used an incidental memory task in humans and obtained valence and arousal ratings during two sessions separated either by 12 h of daytime wake or 12 h including overnight sleep. Recognition accuracy was greater following sleep relative to wake for both negative and neutral pictures. While emotional reactivity to negative pictures was greatly reduced over wake, the negative emotional response was relatively preserved over sleep. Moreover, protection of emotional reactivity was associated with greater time in REM sleep. Recognition accuracy, however, was not associated with REM. Thus, we provide the first evidence that sleep enhances emotional memory while preserving emotional reactivity.

How Best to Optimize Memory

To sleep, to strive, or both: how best to optimize memory
PLoS One. 2011;6(7):e21737. Epub 2011 Jul 20. (Link)
Matthew A. Tucker, Sunny X. Tang, Amaka Uzoh, Alexandra Morgan, Robert Stickgold

While numerous studies have shown that a night of sleep profits memory relative to wake, we still have little understanding about what factors mediate this effect of sleep. A clear understanding of the dynamics of this effect of sleep beyond the initial night of sleep is also lacking. Here, we examined the effect of extrinsic rewards on sleep-dependent declarative memory processing across 12 and 24 hr training-retest intervals. Subjects were either paid based on their performance at retest ($1 for each correct answer), or received a flat fee for participation. After a 12 hr interval we observed pronounced benefits of both sleep and reward on memory. Over an extended 24 hr interval we found 1) that an initial night of sleep partially protects memories from subsequent deterioration during wake, and 2) that sleep blocks further deterioration, and may even have a restorative effect on memory, when it follows a full day of wake. Interestingly, the benefit imparted to rewarded (relative to unrewarded) stimuli was equal for sleep and wake subjects, suggesting that the sleeping brain may not differentially process rewarded information, relative to wake. However, looking at the overall impact of sleep relative to reward in this protocol, it was apparent that sleep both imparted a stronger mnemonic boost than reward, and provided a benefit to memory regardless of whether it occurred in the first or the second 12 hrs following task training.

More faithful memories for those experiences

Awake replay of remote experiences in the hippocampus
Nature Neuroscience Volume 12, Number 7, July 2009
Published online at http://www.nature.com/natureneuroscience
Mattias P Karlsson & Loren M Frank

Hippocampal replay is thought to be essential for the consolidation of event memories in hippocampal-neocortical networks. Replay is present during both sleep and waking behavior, but although sleep replay involves the reactivation of stored representations in the absence of specific sensory inputs, awake replay is thought to depend on sensory input from the current environment. Here, we show that stored representations are reactivated during both waking and sleep replay. We found frequent awake replay of sequences of rat hippocampal place cells from a previous experience. This spatially remote replay was as common as local replay of the current environment and was more robust when the rat had recently been in motion than during extended periods of quiescence. Our results indicate that the hippocampus consistently replays past experiences during brief pauses in waking behavior, suggesting a role for waking replay in memory consolidation and retrieval.

A theory of protoconsciousness

REM sleep and dreaming: towards a theory of protoconsciousness
Nature Reviews| Neuroscience volume 10 | November 2009 | 803
J. Allan Hobson

Dreaming has fascinated and mystified humankind for ages: the bizarre and evanescent qualities of dreams have invited boundless speculation about their origin, meaning and purpose. For most of the twentieth century, scientific dream theories were mainly psychological. Since the discovery of rapid eye movement (REM) sleep, the neural underpinnings of dreaming have become increasingly well understood, and it is now possible to complement the details of these brain mechanisms with a theory of consciousness that is derived from the study of dreaming. The theory advanced here emphasizes data that suggest that REM sleep may constitute a protoconscious state, providing a virtual reality model of the world that is of functional use to the development and maintenance of waking consciousness.

Physiological markers of local sleep

Physiological Markers of Local Sleep
David M. Rector, Jennifer L. Schei, Hans P. A. Van Dongen, Gregory Belenky and James M. Krueger
Eur J Neurosci 2009;29(9):1771-8.

Substantial evidence suggests that brain regions that have been disproportionately used during waking will require a greater intensity and/or duration of subsequent sleep. For example, rats use their whiskers in the dark and their eyes during the light which manifests as a greater magnitude of electroencephalogram (EEG) slow wave activity in the somatosensory and visual cortex during sleep in the corresponding light and dark periods respectively. The parsimonious interpretation of such findings is that sleep is distributed across local brain regions and is use-dependent. The fundamental properties of sleep can also be experimentally defined locally at the level of small neural assemblies such as cortical columns. In this view, sleep is orchestrated, but not fundamentally driven, by central mechanisms. We explore two physiological markers of local, use-dependent sleep, namely, an electrical marker apparent as a change in the size and shape of an electrical evoked response, and a metabolic marker evident as an evoked change in blood volume and oxygenation delivered to activated tissue. Both markers, applied to cortical columns, provide a means to investigate physiological mechanisms for the distributed homeostatic regulation of sleep, and may yield new insights into the consequences of sleep loss and sleep pathologies on waking brain function.